The present invention relates to a method of controlling the density of images reproduced by an electrophotographic process and, more particularly, to an image density control method which forms at least two test patterns on a photoconductive element which have greatly different latent image potentials, detects values associated with the image densities of the two patterns or of toner images corresponding to the two patterns, and controls an image density in response to the detected values.
In an electrophotographic or electrostatic recording apparatus, a latent image is formed a electrostatically on a photoconductive element by a predetermined procedure and the latent image is developed by fine particles of colored toner supplied from a developing unit. Usually, the toner is charged to a polarity opposite to that of the latent image so that it may be electrostatically deposited on the latent image.
A method available for so charging a toner relative to a latent image employs a developer consituted by a toner and a carrier and stirs them together for frictional charging. This type of developer is usually referred to as a two-component developer. While the developing method using the two-component developer is capable of sufficiently charging a toner to a desired polarity, it requires adequate means for maintaining a constant toner concentration in the developer because only the toner is consumed by the development. It is therefore necessary to measure the varying toner concentration in the developer.
For the measurement of a toner concentration, a somewhat indirect method is known as disclosed in Japanese Patent Publication No. 16199/1968. This method comprises the steps of forming a reference latent image pattern electrostatically on a photoconductive drum, developing the reference pattern and photoelectrically measuring the density of the developed image. In a direct method heretofore proposed, on the other hand, the weight or permeability of a developer is measured. Other known methods include one which controls a toner density by detecting a surface potential of a toner image on a photoconductive element (Japanese Patent Laid Open Application No. 92138/1978).
Meanwhile various methods have also been proposed for general image density control purpose such as one which controls the bias voltage for development in accordance with a difference in reflectivity between a reference density plate and an original document (Japanese Patent Laid Open Application No. 103736/1978), one which controls the developing characteristics by detecting an image density during a copying cycle which uses a reference original document (Japanese Patent Laid Open Application No. 141645/1979), and one which controls the amount of charge on a photoconductive element, bias voltage for development and/or illumination intensity by detecting an image density on an original document, latent image potential and toner image density (U.S. Pat. No. 2,956,487).
Of these known image density control methods, those which form light and dark latent image patterns on a photoconductive drum predetermine the timings at which the light and dark pattern areas successively reach a density sensor in accordance with the movement of the photoconductive element. Timed to the start of a scan in an exposure step, pulses synchronous with the rotation of the drum begins to be counted. The density of each of the light and dark areas is read when the count of the pulses reaches a value which represent the arrival of the specific area at the density sensor. Such a method is inapplicable, however, to a copying machine which is capable of various magnifications for image reproduction, without intricate operation for matching the timings with each desired magnification. Besides, inaccuracy may exist in the relationship between the light and dark optical marks on a glass platen and the initial position of a scanning system, while their relative position may even become shifted during operation of the copier. In light of this, the arrival timings of the light and dark pattern areas at the density sensor must be determined so roughly that a substantial range of allowance may be ensured. This in turn requires the light and dark patterns to have wider areas which result in a prohibitively wide image scanning range or erase region.